Mate choice (1 of 2)

When a female wood frog, ready to mate, arrives at a small pond full of singing males, she is jumped by the nearest male, who grabs her around the neck and locks his thumbs together, so he cannot be dislodged. She apparently has little or no choice in the matter—it is first come, first served.

But in many vertebrates, females can and do choose their mates. Sometimes choice is based, at least partly, on the male’s property—his territory, defended against other males and providing nest sites or food sources or protection from weather. For example, male yellow-headed blackbirds carve out territories in a marsh. When females arrive, they cruise around, checking out each male and his property. One of the factors determining their choice of mate is the availability of suitable nest sites at the edges of clumps of cattails or bulrushes—the more edges he owns, the more females he gets.

Males of a wide variety of insects present females with nuptial gifts of food, and females select males on the basis of the size of the gift. The female gains energy for egg production, at least, and if the ability of males to find good gifts is hereditary, she also may get good genes for her offspring.

Sometimes females base their choices on the qualities of the male himself. It might be his song, or the vigor of his courtship dance, or his colors. For example, in the Lower Forty-eight, the size and brilliance of male plumage pattern is the basis for choice by female house finches. Male house finches have red feathers on the head and chest. Females prefer males with intense red coloration and large red chest patches. The red pigment is carotenoid-based, and carotenoids come from the bird’s diet, so the red depends on what the male has eaten (and his ability to convert components of food to red pigment). Thus, house finch females may be choosing males that are the best foragers or have the most efficient metabolism.

A small warbler called a yellowthroat nests in marshes and shrubby swamps. Males have black masks and yellow chests. Careful research has shown that yellowthroat females have marked preferences, but that these preferences differ from region to region: In Wisconsin, females like males with bigger black masks, but in New York, they like ‘em with bigger yellow chest patches. What the females get from making these choices is not clear.

Female preferences also affect the ability of males to obtain extra-pair copulations (many socially monogamous birds engage in very active mating activity outside the pair bond). Mountain bluebirds, for instance, vary in the intensity of blue plumage, and males with brighter blues are more successful in attracting extra-curricular females. Similarly, intensely colored male tree swallows and yellow warblers are preferred by females that copulate with males outside their pair bond.

The bowerbirds of New Guinea and northern Australia have gone a step farther, by transferring the signals to females from themselves to elaborate structures (bowers) that a built solely for the purpose of attracting and courting females. Different species construct bowers in differing shapes—avenues, towers, huts—and decorate them with colorful objects. Each species uses different kinds and colors of objects; some like blue, some like white or yellow, and so on. Within each species, female bowerbirds cruise around and visit the bowers of the males and judge each male on the construction and decoration of his bower. After she makes her choice and mating takes place, she goes off to build a nest and rear chicks by herself.

What’s the payoff to females for making these choices? In some species, there is a direct benefit in terms of resources such as nest sites or food. In other cases, males with brighter colors turn out to be better providers for the offspring. When males do no parental care, females might at least get good genes for their offspring. And in other cases, choosing a ‘sexy’ male might mean that the sons of the choosing female will also be sexy and successful in attracting females. The so-called sexy-son hypothesis obviously requires that the attractive characteristics are genetically based and inheritable.

Of course, male animals make choices too. Males of some species choose on the basis of color or pattern, just as females do. For instance, for whatever reason, male barn owls prefer to mate with females that have lots of lovely black spots on their white breasts. However, in many vertebrates, males seem to be somewhat less choosy than females.

When female creatures choose males, we do not suppose that the choices are conscious in the human sense. (Nor can we say that all human female choices are necessarily conscious!). All that is required is that there is variation among males and that females can discern the differences and act accordingly.

The result of mate choice (and of the competition to be chosen) is, in the big picture, an incredible diversity of color and form in the animal kingdom—diversity that is not directly related to making a living or simple survival but, instead, is related to mate selectivity and being attractive to the opposite gender.

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Mate choice

Last week I wrote about female animals making choices of mates on the basis of the male’s property, nuptial gifts, songs and dances, or physical appearance. Those choices occur before copulation (in animals with internal fertilization: sperm meets egg inside a female) or spawning (in those with external fertilization: sperm meets egg outside the female). However, there are also some much subtler forms of female mate choice that have been revealed by recent research. These choices occur during or after copulation or spawning.

Here are a few examples: In some fruit flies, sperm size varies and females discriminate against small sperm—and thus against males that make small sperm. In ducks and fruit flies, preliminary studies suggest that females may be able to kill sperm that come from copulation with unwanted males. Females can eject sperm after mating in certain damselflies, flour beetles, flies, and songbirds, ridding themselves of undesired sperm and leaving themselves then available for a preferred mate.

One of the best-studied examples comes from a free-ranging population of chickens, of a breed similar to the red jungle fowl that is the ancestor of domestic chickens. Roosters maintain a strong dominance hierarchy among themselves, and hens prefer to mate with high-ranking roosters. These males provide hens with better courtship feeding, protection from predators, and protection from sexual harassment by subordinate males. Dominant males leave more descendents than subordinate males, because they mate more often. Dominance is heritable, so their sons are likely to be preferred partners also.

Subordinate males, however, obtain some copulations by forcing the hens to mate, even when the hens resist. When accosted by a subordinate male, hens make a distress call, which induces dominant males to disrupt the copulation attempt. In addition, even if the subordinate male achieves copulation, hens can eject his sperm immediately, even before he dismounts from her back. Then she is free to mate with a preferred, dominant male and the dominant male’s sperm can fertilize her eggs.

When female salmonid fishes release their eggs during spawning activity, the eggs are accompanied by ovarian fluids that can affect sperm swimming speed, longevity, and motility. Arctic charr females can change the composition and effect of their ovarian fluid, depending on just what male is trying to fertilize her eggs. Different females of rainbow trout may produce ovarian fluid with differing effects on sperm motility, and thus some females may be able to be choosier than others.

It is even possible that plants may be able to mate selectively with certain individuals. It is well known that many plants are self-incompatible—that is, they reject pollen from themselves or from genetic relatives (which share their genes). But, in addition to this common form of selectivity, experiments have indicated that some plants discriminate among the pollen grains of different unrelated pollen donors (males), thus allowing only certain males to fertilizes the ovules.

These kinds of subtle interactions that occur during or after mating have only been discovered rather recently. Many more possibilities in this hot research area are now being studied in a wide variety of organisms. The take-home message from these studies is that females are not necessarily passive receptacles for sperm, but at least in some cases they are able to exert what is called cryptic female choice.